For Release January 30, 2001

AGRI-VIEWS
by Chuck Otte, Geary County Extension Agent

With the recent jump in nitrogen fertilizer prices, many crop producers are starting to ask a lot of questions about optimum nitrogen fertilizer rates. In some cases it isn’t even a case of what’s the optimum, but what’s the least I can put on without hurting yield.

There are a lot of factors that go into determining a proper fertilizer rate. What crop are you growing? What kind of soil do you have and how much usable rooting depth is there? What are the chemical characteristics of the soil, meaning how much residual nutrients are there, how much organic matter and what’s the soil pH? Is the field irrigated or dryland and what’s the target yield, or what’s the yield history of that crop in that field?

There are interactions between the various nutrients. You may be most concerned about nitrogen, because of the cost, but if phosphorus is the limiting factor in your soil, that has to be taken into account. Since last year was a dry year, there may be more residual soil nitrate than normal. But we won’t know without a good profile soil sample, 2 feet deep or the effective rooting depth, whichever is shallower, for analysis. High organic matter soils will release more nitrogen into the soil. We can’t measure that directly though, we just have to use a rule of thumb based on soil organic matter levels.

There are also several economic factors we take into consideration when making optimum recommendations. We know that with every pound of added nitrogen fertilizer, yield will increase to a maximum yield level and then level off or start to decline. We also know that the economic return from that added fertilizer will taper off long before the agronomic yield will taper off, that old law of diminishing returns.

So, from all of this, you want me to tell you, off the top of my head, what the optimum or minimum nitrogen fertilizer rate is for your field? Well, just remember that my answer is only as good as the information you provide to me! What do we know for certain for starters? Dryland corn and milo (grain sorghum) need about 1.25 to 1.3 pounds of nitrogen for every bushel of grain they will produce. Irrigation will add .05 to .1 pound per bushel. Wheat will need about 1.75 pounds of nitrogen per bushel. We don’t have a factor for soybeans. Since soybeans are a legume and are supposed to symbiotically fix atmospheric nitrogen, we don’t normally apply nitrogen fertilizer to soybeans. (You can ask me my theories on this sometime.)

But don’t immediately go out and decide that you need to apply 125 pounds of nitrogen for that 100 bushel target yield. We still have to take into account credit for previous legume crops grown in the past two years. We have to take into account any credits for manure applications, and under ideal conditions we would have soil nitrate levels from a soil test. What about the diminishing returns factor? Well, in a nutshell, I can’t give you an answer. With the soil variability that we have in this area you can have a range of values that are meaningless. Each situation will be different and we’ll evaluate them on a case by case basis.

How critical is the soil nitrate test? Let’s say that you found that in the two foot soil profile there was 10 parts per million residual nitrate. Even assuming that half of that was going to be available for crop growth, you could reduce nitrogen fertilizer rates by at least 35 pounds. Using the cheapest nitrogen fertilizer currently available, you just saved $8.50 per acre. The soil test cost you about $10 for the whole field. Everyone wants to save money on crop production. But the soil test is probably the most critical part of the equation. Don’t guess, soil test!

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